1. Field of the Invention
The present invention to relates to a method of producing a metal insulator semiconductor (hereinunder referred to as "MIS") type semiconductor device having a doped channel structure.
2. Description of the Prior Art
In an MIS type semiconductor device having a doped channel structure, the threshold gate voltage is generally adjusted by controlling the amount of ions implanted for channel doping with respect to the impurity density of the impurities of the substrate. That is, channel doping for controlling the threshold voltage by selectively implanting the ions of impurities in the channel region of an MIS transistor makes best use of the good controlling property of low-density doping. In this case, the threshold voltage is determined in accordance with the supply voltage, and the lower limit value of the threshold voltage is determined in accordance with the consumption of current during a rest, i.e., when the device is in the operating state in which it experiences minimum power consumption. However, the amount of channel doping ions implanted for adjusting the threshold voltage is determined not in consideration of the swing, which exerts a great influence on the consumption of current during a rest, but merely in conformity with the threshold voltage value. The term "swing" refers to the change in the gate voltage which is required for changing the value of the drain current by one order of magnitude, i.e., by a factor of 10, in the region below the threshold voltage.
The swing and the drain current flowing when the gate voltage is 0 V, namely, the cut-off current, are explained in detail in the text Physics of Semiconductor Devices by Sze. S. M., published by John Wiley Interscience (1981).
If the quantity of ions implanted for channel doping is determined only on the basis of the threshold value without taking the swing into consideration, as in conventional manufacturing methods, the swing takes on a large value and the cut-off current value increases, thereby greatly increasing the consumption of current during a rest. This tendency is prominent in an MIS type transistor having a short channel.
In addition, since demand for reduced power consumption has recently become strong, as well as demand for ICs of higher integration and higher capacity, reduction in the consumption of current during a rest by reducing the cut-off current has become a very important aim to be achieved.